When printing multi-color images, accurate registration is required between colors. Since most containers have neither accurate reference features nor stiffness, it is difficult to print multi-color images on them. Such printing normally requires multiple printing units (one for each color) and registration is difficult to maintain when a container is transferred between successive print units. For this reason most color images on bottles are done by applying a pre-printed label to the bottle, increasing production costs over direct printing. In some cases, such as when printing on drinking cups or unfilled cans, a mandrel may be inserted into the container to achieve stiffness and registration (see for example U.S. Pat. Nos. 5,193,456 and 3,661,282), but, in the great majority of cases, the insertion of a mandrel to fill the container and allow registration is not possible at all, as it requires an opening at least as large as the largest cross-section.
The flexographic printing process is an ideal process for printing on thin-walled containers as it requires almost no pressure, so a method of utilizing flexographic printing on containers is highly desirable. A typical flexographic press comprises an ink supply (also referred to as an “ink fountain”), a metering roll in contact with the ink supply and transferring an accurately metered amount of ink to the plate (which is mounted on a plate cylinder), a material to be printed, usually in the form of a web, and an impression cylinder used to support the web. The most common form of metering roll is known as an anilox roll, which is a hard cylinder engraved with a continuous pattern of small pits. The excess ink is removed by a doctor blade or a reverse roll, leaving ink only in the recessed areas. The flexographic plate operates in a manner similar to the common rubber stamp: the elevated areas are inked and this ink is transferred to the web. The plate is usually mounted on a thin layer of cushioning foam.
It is an object of the invention to allow direct flexographic printing of monochrome and color images directly onto containers such as plastic and glass bottles, cans, cups, jars, and the like. It is a further object to address the registration problem in a manner compatible with present flexographic press design.